We calculate the spectral line shape of an isolated line from first principles, assuming that the translational motion is not statistically correlated with the evolution of the optical coherence, i.e., with the broadening. We use the known, realistic potentials for the influence of collisions on the translational motion and on the internal motion. We show that the calculated profiles do not agree, particularly at low pressures, with very precise experimental profiles of the P(2) line of CO in a bath of Ar. We establish that the source of the disagreement lies in the assumption of uncorrelated effects of collisions on the translational motion and the optical coherence associated with the internal degrees of freedom.

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Newly developed quantum cascade laser spectroscopic instrumentation allows high precision measurements of atmospheric trace gases, including measurements of small fractional (<10−3) changes in stable gases (e.g. N2O), and isotopic ratios (e.g. 13CO2/12CO2, 13CH4/12CH4).